Modulated arm width spiral antenna

ABSTRACT

A multi-arm spiral antenna which allows unlimited broadband operation with dual senses of circular polarization. The antenna comprises spiral arms having width variations which are logperiodically scaled to produce local reflection (stopband) regions along the arms. The position of the stop-band regions is a function of the period and amplitude of width variations. Arm currents are produced by excitation of the antenna. These currents are reflected by the stopband regions. The relative phase of the reflected currents is a function of the relative scaling of the arms.

United States Patent Ingerson [451 Aug. 1,1972

[54] MODULATED ARM WIDTH SPIRAL 3,562,756 2/1971 Kuo et a1. ..343/895Primary Examiner-Eli Lieberman Attorney-Daniel T. Anderson, AlfonsValukonis and Harry L. Jacobs 57] ABSTRACT A multi-arm spiral antennawhich allows unlimited broadband operation with dual senses of circularpolarization. The'antenna comprises spiral arms having width variationswhich are log-periodically scaled to produce local reflection/stopband)regions along the arms. The position of the stop-band regions is afunction of the period and amplitude of width variations. Arm currentsare produced by excitation of the antenna. These currents are reflectedby the stopband regions. The relative phase of the reflected currents isa function of the relative scaling of the arms.

11 Claims, 10 Drawing Figures PATENTEnws' 1 1912 3,681- 772 sum 1 or 3Fig. la

I Fiqlb PRIOR ART I Paul G. lngerson VVIENTOR,

AGENT mm 1 m2 3.681.772

' sum 2 or 5 Paul G. Ingerson INVENTOR.

AGENT PATENTEmus r1912 3.681.772

Paul G. Ingerson INVEJNTOR.

Fig.70 FiqTb tgwfmz AGENT 1 MODULATED ARM WIDTH SPIRAL ANTENNABACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to spiral antennas and specifically to multi-arm spiral antennahaving both left-hand and right-hand circular polarizationsimultaneously produced over wide bandwidths. 2. Description of thePrior Art The customary method of feeding broadband spiral antennas atthe center terminals yields a radiation field which is ellipticallypolarized. If the parameters of the antenna are chosen properly, theradiation polarization can be made almost circular. t

The sense of circular polarization (CP) is said to be right-handedcircular polarization (RHCP) if the electric field vector rotates in thedirection of the fingers of the right hand when the thumb points in thedirection of propagation. correspondingly, left-handed circularpolarization (LHCP) is defined by an electric field vector rotating inthe direction of the fingers of the left hand when the thumb points inthe direction of propagation.

Spiral antennas are said to have a sense of wrap. The spiral sense ofwrap is, in accordance with general practice, determined by the handused when pointing the fingers in the direction of the arm current andthe thumb in thedirection of propagation of the radiated fields.Broadband operation of a spiral antenna yields the sense of polarizationof the radiated field determined by the sense of wrap of the spiral.

Many applications of extremely broadband antennas require the ability toreceive both RI-ICP and LHCP signals simultaneously over the entirebandwidth in a single antenna. One example where such a single antennawould be required is for the feed of a parabolic reflector. If thesystem is to be capable of receiving or transmitting both RHCP andLI-ICP, as well as any linear polarized signal without a loss in gaindue to polarization, then the single feed of the parabolic reflectormust be capable of RI-ICP and LHCP operation. Presently, such receivingor transmitting capability, has not been possible with the whole classof frequency independent spiral antennas. Some narrow band techniquesare used whereby spiral antennas can be fed from the outside of thespiral in addition to feeding from the center. Since the direction ofthe current relative to the direction of propagation when fed from theoutside is opposite to the direction of current, for the same directionof propagation when fed from the center, both senses of circularpolarization can be obtained from a single spiral. This dual sense ofpolarization operation is bandwidth limited. If the ratio of the upperto lower frequencies of operation is used as a measure of bandwidth,this ratio will be less than to l for the method of obtaining dualsenses of CP from a single spiral antenna, where the antenna is fed fromboth the outside and center.

The present invention provides a new design of spiral antenna requiringonly a center feed and which is capable of simultaneous dual circularpolarization operation over any chosen bandwidth limited only bypractical construction considerations in the feed region.

. BRIEF SUMMARY OF INVENTION This invention relates to a multi-armspiral antenna which is fed at the center of the spiral. Each of thespiral arms of the antenna have a cascade of cells, with each cellhaving a wide and narrow section along the lengths of the arms. Thelengths of the sections increase for increasing distance from thecenter.

BRIEF DESCRIPTION OF DRAWINGS 1 FIG. 1a shows an elevation view of aconventional 2 arm equiangular spiral;

FIG. lb shows an enlarged plan view of the top portion of theconventional 2 arm equiangular spiral;

FIG. 2 shows an elevation view of a 2 arm equiangular modulatedarm-width spiral embodying the invention;

FIG. 3 is an enlarged plan view of the top portion of the 2 arm spiralwidthself-complementary modulated arm width as shown in FIG. 2;

FIG. 4 is a plan view of a 4 arm equiangular modulated arm width spiral;

FIG. 5 illustrates the coordinate system;

FIG. 6a and 6b show typical radiation patterns of the electric field fortwo modes (M and M of feeding a four arm spiral using conventionalantennas;

FIGS. 7a and 7b show the radiation patterns in the same modes of feeding(M and M as represented in FIG. 6 using the antenna of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS; la and lb show aconventional two arm conical spiral antenna 19. comprising twospiralarms 20 and 21 which are wound on the surface of an electricallynonconductive cone 22. A balanced feed line 23 is located at the coneaxis of symmetry. The balanced feed line 23 comprises two coaxialtransmission lines, 24 and 25. The outer shields of the coaxial lines 24and 25 are electrically connected together in FIG. lb, along theirlengths. The center conductors 26 and 27 of the lines 24 and 25 areelectrically connected to the feed terminals 28 and 29, respectively, asshown inFlG. lb.

The parameters which define the equiangular conical spiral are alsoshown on FIG. 1a. These are: l 0/ 2, the half-angle of the cone (2) a,the angle of spiral wrap, which is the angle between a tangent to theedge of an arm and a line joining the tangent point and apex of thecone, and (3) 6, the angular width of the spiral arms, found by theangular rotation needed to rotate the spiral defined by one edge a ofthe arm into congruence with the opposite edge b.

The customary method of feeding a broadband twoarm spiral at the centerterminals yields two'modes of operation whose sense of polarizationdepends on the sense of wrap of the spiral. In accordance with generalpractice, the polarization sense of the spiral antenna is determinedfrom the hand used when pointing the fingers in the direction of thespiral arms current and the thumb in the direction of propagation of theradiated fields. Since the currents are assumed to travel away from theinput temiinal, right-hand'circular polarization (RHCP) corresponds toprogressive phase delay of the arm currents in the increasing qbdirection; left-hand circular polarization (LI-ICP) to progressive phaseadvance of the arm currents in the decreasing dz direction; where d; isthe conventional polar coordinate and increases in the counter clockwisedirection as shown in FIG. 5.

The push-pull mode, referred to herein as Mode l, is obtained when thetwo spiral arms are fed l80 out of phase. The radiation patterns of Model are single beam patterns, which in the case of conical spirals areundirectional and directed along the axis in the direction of the apexof the cone.

The push-push mode, referred to herein as Mode 2, is obtained when thearms are fed in phase, with a center post formed by the outer shields ofthe two feed lines, fed 180 out of phase.

The radiation pattern of Mode 2 is a toroid about the axis of the cone.The characteristic of this mode is a null in the radiation pattern alongthe axis of the spiral.

For analysis, the antenna is divided into three re gions, thetransmission region, the active region and the unexcited region. In thetransmission region the arm currents travel'along each arm withessentially free-space phase velocity and negligible radiation. Theactive region corresponds approximately to the region where the phasedifference in the currents in the arm allows substantial radiation. Ifthe active region is sufficiently wide to allow substantially totalradiation of energy carried by the arm currents, the portion of the armsfollowing the active region are essentially unexcited and henceconstitute the unexcited region in a properly operating frequencyindependent antenna.

In Mode I (M the active region is approximately located at a diameter ofthe cone (D) of 'y/rr (where 'y is the wavelength at the givenfrequency) so that the currents in the arms are phased approximately anextra 180 each half turn and, hence, are in phase.

In Mode 2 (M the active region does not occur until D 2'y/1r. Hence,when the spirals diameter is smaller than 2'y/1r, the incident energyinto Mode 2 is not efficiently radiated.

FIG. 2 shows an embodiment of the invention which is a multi-arm conicalspiral antenna having two arms. The arms 30 and 31 are wound on anelectrically nonconductive cone 32. The arms, however, have periodicvariations in the conductor arm width, called modulation. The arms ofthe antenna with such periodic variations can be thought of as beingconstructed from a cascade of cells, with a cell, for example 33, havinga wide section, 34, and narrow section 35. Amplitude is defined as theratio of the width of the wide section 34 to the width of the narrowsection 35. The period is defined as the ratio of the lengths ofadjacent cells. The lengths of the sections increase for increasingdistance from the feed terminals 40 and 41 at the apex of the cone shownin FIG. 3. The antenna is fed by two coaxial lines 36 and 37, located onthe axis of symmetry, whose outer shields are electrically shortedtogether along their lengths, as shown in FIG. 3. The center conductors38 and 39 of the two lines 36 and 37, are connected at the top of thecone to the arms 30 and 31 at the terminals 40 and 41, respectively.When the arms 30 and 31 are fed 180 out of phase (Mode l), the currentsalong the center post cancel each other. The Mode 1 method of feedingyields single beam patterns, which are essentially unidirectional anddirected along the axis in the direction of the apex of the cone, aswith the conventional unmodulated arm width spiral, as illustrated inFIG. 1. In Mode 2 the arms are fed in phase with the center post 42formed by the outer shields of the coaxial lines 36 and 37 fed 180 outof phase.

The energy in Mode 2 (M generally will not radiate efficiently until thecircumference of the spiral is approximately 2 wavelengths. By makingthe variations in the arm width sufficiently large, it is possible toform reflection regions along the arms such that essentially all theincident energy along the arms is reflected by the arm impedancemismatch caused by these variations.

The maximum reflection along the arms is found to occur in that regionwhere the length of the cells becomes approximately /y. Further, thereflection from cells which are shorter than re-y is small. Hence, thereflection of the incident energy along the arms is confined to a regionof the arms where the cells are approximately /y long. This region iscalled a stopband region. By choosing the proper parameters of amplitudeof the variations and the period of the variations, the stopband regioncan be placed at any desired diameter.

In the two-arm spiral case the stopband would be placed between theactive regions of the M and M modes. Hence, the normal M mode would beunaffected if the M active region is efficient. The placing of thestopband ahead of the active region for the M mode assures that thestructure when fed from the center in the M mode will not radiateefficiently, since the energy is reflected before reaching the requiredactive region for substantial radiation.

If, further, the modulation of the arms is complementary, in the sensethat the regions of modulation are opposite in the two arms atcorresponding points, then the reflected energy will be 180 out of phasebetween the two arms. This is then the condition for substantialradiation of the reflected energy in the opposite sense of polarizationto Mode l as discussed. If the spiral is an equiangular (logarithmicspiral), then log-periodically scaling the lengths of the cells, allowsthis region to move along the structure retaining its relative positionbetween the active regions of the M and M modes. The self complementarygeometry makes the relative phase of the reflected arm currents 180 outof phase,

independent of frequencies. Hence, the antenna will now have broadbandoperation with both senses of circular polarization in a simple singlebeam, the M mode giving one sense of CP and the M mode the opposite.

In the general case of an N arm antenna, the phase change betweensuccessive arms would be whereM=1...N.

In. general, only those choices which suppress radiation from the centerfeed post are used. Hence, M N is in general not used since the armswould be fed against the feed post for this mode.

For a multi-arm spiral we can identify the modes by the excitation ofthe arms, and hence for M l, 2, 3 we will refer to these as M M and Mmodes of excitation, respectively.

The principles may be applied to a multi-arm spiral having 4 spiral armsas follows. Assume the four arm spiral is initially a LH wound spiral asshown in FIG. 4. The normal feed for LHCP sum pattern (Mode l) is A =(O,90, l80,+ 90) where A 32 (I I I I is the current vector notation for thephase of the excitation at the four input terminals 51, 52,53, 54.

For the M 1 Mode (M the four arm spiral gives a typical pattern shown inFIG. 6A. For the M l mode (M of the four arm spiral, the active regionstill occurs at a diameter (D) of about 1 wavelength. The M 3 Mode (Mhas the excitation:

The active region for this mode will occur approximately at a diameterof 3 wavelengths. When the antenna is large enough (D 3y) to supportsubstantial radiation from this mode, the antenna gives a typicalpattern shown in FIG. 6B.

If, however, the structure is not large enough to suP- port substantialradiation of the M mode, simple reflection of the current at the ends ofthe structure 55, 56, 57, 58 produce a return excitation whichefiiciently radiates like the M mode, i.e. a smooth lobe pattern, butwith the opposite sense of polarization.

Hence, introducing a modulation in the arms to effect a stopband regionwith the proper relative phasing between the arms after the M 1 activeregion (i.e. at a diameter larger than 1y), but before the normal M 3active region, is sufficient to give the required dualpolarizationoperation. With the relative phase of the reflected arm currents thesame (all the reflection coefficients the same) it is possible to obtainthe opposite polarization for the M mode; since, however, the excitationvector, A =(0, 0, 0,0) or 180, 180, 180, 180) is not useful if themodulation is scaled the same in each arm so that the arms areidentical, a six arm spiral must be used to obtain multi-mode operationwith dual polarization.

While the theory of log periodically modulating the arm width of spiralantennas to effect stopband regions does not depend upon the proximityof the modulated sections between each arm, the actual construction forplanar and conical spirals appears to be optimized by using aself-complementary arrangement i.e., a structure in which the metal areais identical in size and shape to the open area. This leads to dividingeach turn of the spiral into 2N equiangular segments. In the alternatesections the arm widths are made wide or narrow. A cell is, as definedbefore, composed of two sections of line-one section of wider width andone narrower. Since the maximum reflection along each arm occurs whenthe total length of a cell is approximately 'y long, the number of cellsin circumference of the spiral in the region of maximum reflection willbe N 1,2,3, etc. By selecting N to be an integer, all the wide or narrowsegments will lie in pie shaped wedges, shown as 70 77 in FIG. 4,allowing the most interaction of the wide portions of each arm, sincethey are then closest to each adjacent arm s wide segments over thelongest distance. Correspondingly, the narrow portions have the leastinteraction over the longest distance. This then should allow thesmallest modulation for a given size stopband region since the size ofthe stopband region in the periodically modulated transmission linestructure appears to be maximized when the change between the twoimpedance levels is a step function.

Thus, for the two arm spiral where the modulation of the arms themselvesis to be opposite to give the reflected energy a relative phase shift,the number of cells in circumference must be odd. This guarantees thatthe two arms will be physically complementary to each other. For a fourarm spiral, using the M l and M 3 modes to obtain dual polarization, thearms will all be the same and, hence, an even number of cells will beused.

Since the location of the maximum reflection region occurs when the celllength is approximately /z'y and the size of the stopband region iscontrolled by the amplitude of the modulation, it is apparent that thelocation and size of the stopband region can be selected by parameterchoice.

The value of N is to be chosen so that the reflection region occursbetween the active region of the selected modes on unmodulated armwidthspirals. Hence, in the two arm case, N 3 seems appropriate while in thefour arm case N 4 for the M l and 3 modes. This physical arrangement wasused in FIGS. 2, 3 and 4 it is believed to be the optimum. The ratio ofthe angular width of the wide section of a cell to a narrow section isdefined as the modulation ratio. It has been found that ratios of fourare sufficient to form the necessarY stopband regions when the otherparameters of the spiral, i.e. 0 and a, are selected to give goodperfonnance of the regular unmodulated spiral.

We claim:

1. A multi-arm spiral antenna capable of radiating circularly polarizedelectromagnetic energy with opposite senses of polarization, saidantenna comprising:

a. at least two spiral arms, each arm consisting of a plurality ofinterconnected cells, each cell consisting of a wide and a narrowsection along the length of its arm, the transition between adjacentwide and narrow sections being relatively abrupt, whereby to reflectelectromagnetic energy, the sections of each cell increasing in lengthas the distance from the center increases, and

b. means for feeding said arms at the center of the spiral withelectromagnetic energy.

2. The antenna of claim 1 wherein:

the ratio of the lengths of adjacent arm-cells is constant.

3. The antenna of claim 2 wherein:

the ratio of the widths of the wide section of adjacent arm-cells isconstant.

4. The antenna of claim 3 wherein:

the ratios of the widths of the wide sections of adjacent cells and thecell lengths of adjacent cells are equal.

5. The antenna of claim 3 wherein:

the ratio of the width of the narrow section of adjacent arm cells isconstant.

6. The antenna of claim 4 wherein:

the ratios of the narrow sections of each cell are equal to the ratiosof the cell lengths.

7. The antenna of claim 1 wherein:

the said spiral is an equiangular spiral.

8. The antenna of claim 7 wherein:

the number of said arms is four.

9. The antenna of claim 8 wherein:

the arms are identical in size and shape.

10. An antenna as defined in claim 1 wherein the spacing betweenadjacent ones of said arms is nonuniform, thereby to provide a change ofimpedance in a direction along said arms.

11. An antenna as defined in claim 1 wherein said 5 arms are disposed onthe surface of a cone.

1. A multi-arm spiral antenna capable of radiating circularly polarizedelectromagnetic energy with opposite senses of polarization, saidantenna comprising: a. at least two spiral arms, each arm consisting ofa plurality of interconnected cells, each cell consisting of a wide anda narrow section along the length of its arm, the transition betweenadjacent wide and narrow sections being relatively abrupt, whereby toreflect electromagnetic energy, the sections of each cell increasing inlength as the distance from the center increases, and b. means forfeeding said arms at the center of the spiral with electromagneticenergy.
 2. The antenna of claim 1 wherein: the ratio of the lengths ofadjacent arm-cells is constant.
 3. The antenna of claim 2 wherein: theratio of the widths of the wide section of adjacent arm-cells isconstant.
 4. The antenna of claim 3 wherein: the ratios of the widths ofthe wide sections of adjacent cells and the cell lengths of adjacentcells are equal.
 5. The antenna of claim 3 wherein: the ratio of thewidth of the narrow section of adjacent arm cells is constant.
 6. Theantenna of claim 4 wherein: the ratios of the narrow sections of eachcell are equal to the ratios of the cell lengths.
 7. The antenna ofclaim 1 wherein: the said spiral is an equiangular spiral.
 8. Theantenna of claim 7 wherein: the number of said arms is four.
 9. Theantenna of claim 8 wherein: the arms are identical in size and shape.10. An antenna as defined in claim 1 wherein the spacing betweenadjacent ones of said arms is nonuniform, thereby to provide a change ofimpedance in a direction along said arms.
 11. An antenna as defined inclaim 1 wherein said arms are disposed on the surface of a cone.